A. Field of the Invention
This invention relates to the field of art of thin film photovoltaic solar cells.
B. Background Art
A photovoltaic solar cell, often referred to as a solar cell, is a semiconductor junction device which converts light energy into electrical energy. A typical photovoltaic solar cell is a layered structure comprising four principal layers: (1) an absorber-generator, (2) a collector-converter, (3) a transparent electrical contact, and (4) an opaque electrical contact. When light is incident upon the absorber-generator, the device generates a voltage differential between the two contacts and an electrical current which increases as the intensity of the light increases.
The absorber-generator (referred to as the "absorber") is a layer of semiconductor material which absorbs light photons and, as a consequence, generates minority carriers. Typically, the absorber captures photons and ejects electrons thus creating pairs of negatively charged carriers (electrons) and positively charged carriers ("holes"). If the absorber is a p-type semiconductor, the electrons are minority carriers; if it is n-type, the holes are minority carriers. As minority carriers are readily annihilated in the absorber by recombination with the plentiful majority carriers, they must be transported to a region where there are majority carriers before they can be utilized by an electrical circuit.
The collector-converter (the "collector") is a layer of material in electrical contact with the absorber wherein the majority carriers are of the same conductivity type as the minority carriers generated in the absorber. This layer "collects" minority carriers from the absorber and "converts" them into majority carriers. If the collector is an oppositely doped region of the same semiconductor as the absorber, the photovoltaic device is a p-n junction or homojunction device. If the collector is comprised of a different semiconductor, the device is a heterojunction; if the collector is an insulator and a metal, the device is a metal-insulator-semiconductor device and, if the collector is metal, the device is a Schottky barrier device.
To utilize the photovoltaic properties described above, one needs to add electrical contacts. In general, one of the electrical contacts is transparent while the other is opaque. Either the opaque or transparent contact may also serve as a substrate.
Two other functions are usually added to solar cells: encapsulation to improve the durability and anti-reflection to increase the number of photons which penetrate into the device (rather than be reflected).
The central characteristic of the encapsulant is that it protects the solar cell from the environment. One side of the solar cell must have an optically transparent encapsulant.
In the case where the encapsulant is applied over the transparent contact, glasses have proven to be most successful. These glasses can be formed from a broad range of compounds based on silicon, oxygen and other elements. In systems commonly used today, these glasses are bonded to a solar cell or module after fabrication, but development of integral encapsulants in the range of 5 microns thick directly applied to the solar cell is becoming a practice.
The opaque contact usually serves as the encapsulant for the surface that is not facing the source of light.
All of the semiconductor material systems under study for solar cells have high indices of refraction resulting in reflection from a planar surface in the range of 25 to 40 percent. In order to prevent these high reflection losses, anti-reflection layers are necessary.
There are two primary approaches to the reduction of reflection losses. Texturing of the surface of the semiconductor causes multiple reflections for incoming photons, reducing the net photon loss. Single or multi-layer, anti-reflection coatings reduce reflection by both index matching and interference effects. A variation of the first approach is to build a textured layer into the encapsulant. Various combinations have been successfully utilized including texturing the semiconductor and providing an anti-reflection layer on top of this material.
Some of the design requirements for the type of cell described above are provided by Barnett and Rothwarf, "Thin-Film Solar Cells: A Unified Analysis of Their Potential" in IEEE Transactions on Electron Devices, Vol. 27, No. 4, April 1980.
Most of the solar cells being manufactured today evolved from the silicon semiconductor industry. These solar cells are made from wafers sliced from ingots. Improvements in ingot information, productivity, and improved wafer sawing techniques are being developed.
The lowest cost solar cells are made by depositing thin films of semiconductors on low cost substrates. These thin layers are designed to reduce the consumption of semiconductor material by 80% to 95% or more.
In particular, as described in the Nelson U.S. Pat. No. 3,565,702, issued Feb. 23, 1971, which disclosure is incorporated herein by reference, thin films can be deposited using a sliding boat, liquid phase process which deposits successive epitaxial semiconductive layers on a substrate from the liquid phase. This is performed, for example, by bringing one surface of the substrate in contact with a first solution consisting of a first molten metallic solvent in which a first semiconductive material is dissolved and then cooling the first solution in order to deposit a first epitaxial layer on the substrate. After the first layer is grown, the steps are performed again to deposit a second epitaxial layer. Alternately, it is well known in the art that the second layer may be formed by passing the substrate with some of the liquid film covering it through a dopant gas which will dope the second layer. It is known that the second layer can also be formed by diffusion or ion implantation.
Thin film silicon solar cells have been obtained by using other deposition techniques, such as vapor (CVD, vacuum or sputtering), and molten liquid. With the exception of CVD on single crystal metallurgical grade silicon, these techniques have not produced high performance solar cells with low silicon utilization. One of the greatest drawbacks has been the high material costs encountered in the preparation of single crystal silicon solar cells. Preparation of thin films by way of vapor deposition on non-silicon substrates also tended to produce small grains which led to minority carrier recombination at the grain boundaries or other effects which limited the diffusion length and hence, reduced the collection of minority carriers and their conversion into majority carriers, thereby reducing the current. This has led to the use of molten liquid growth on foreign substrates which has demonstrated large grains but considerable surface and bulk contamination. Also, thicknesses of 100 microns or more seem required. Grain boundary recombination has also led to reduced currents.
Accordingly, polycrystalline silicon has not been demonstrated with high efficiency in a low-cost thin film configuration on non-silicon substrates. The limited efforts which used a low-cost configuration have reported efficiencies under five percent. The higher efficiencies for polycrystalline silicon are based on devices made on higher cost substrates and with much thicker films.
An object of this invention is to drive the growth of the semiconductor, on a metallurgical barrier layer for med on the substrate, by the application of a gradient (either temperature or electric field) across the melt with the substrate being cooler such that the whole apparatus including the melt is not cooled during the growth phase of the liquid phase epitaxial process.
It is an object of this invention to provide an optically reflective barrier layer between the substrate and the first semiconductor layer in order to increase the performance of the cell.
It is also an object of this invention to provide a method of making thin film photovoltaic solar cells which is of extremely low-cost and yet produces high light-to-electrical energy conversion efficiency.
Another object of this invention is to provide a method of continuous or semi-continuous semiconductor deposition which permits the growth of extremely large grains.
Yet another object of this invention is to provide a method of depositing extremely thin, large grain semiconductors on a low-cost substrate.
Yet another object of this invention is to provide a method of depositing large grain semiconductors which do not have contamination at the grain boundaries.